Peroxisomes are intracellular organelles whose biogenesis and function are linked to many human health concerns, including inherited neuropathologies, aging, cancer, heart disease, obesity and diabetes. Moreover, the size, number and content of peroxisomes in a cell are regulated and can change dramatically in response to factors such as fats, hypolipidemic drugs, carcinogens and cell differentiation. Understanding the fundamental biology of peroxisome biogenesis and function is critical to our understanding of these conditions and to their future treatment. To this end, many groups have exploited yeast genetics to identify and characterize a class of proteins -termed peroxins - important to peroxisome biogenesis. Our group has recently extended these studies by applying innovative, comprehensive and systematic approaches to identify novel components of the biogenesis program. Whole-genome transcriptional arrays identified mRNAs that increase in abundance in response to conditions that induce peroxisomes and two state-of-the-art complementary proteomics approaches applied to isolated peroxisomal subcellular fractions identified several previously uncharacterized putative peroxisomal proteins. The objective of this proposal is to characterize a relatively small group of novel proteins identitfied through these procedures and selected based on their expression patterns and their presence in (or on) peroxisomes. The roles of these proteins in the formation and maintenance of the organelle will be determined. A thorough examination of each protein, using combinations of current in vitro, in vivo, morphological and genetic approaches is proposed. Preliminary results applying these techniques demonstrate that the first these proteins is a previously unknown peroxin required for peroxisome size regulation and segregation during cell division.